Gesundheit für Mensch, Tier & Pflanze

Grain weevil

Sitophilus granarius

Profile

The grain weevil is one of the most important storage pests of grain stocks and is typical of warehouses and small farms. As a widespread grain pest, its infestation has a major economic impact in agriculture.

Appearance

The cylindrical beetles, measuring about 3.5 to 5 mm, are somewhat smaller than cereal grains. Their coloration changes from light brown to dark brown to blackish brown depending on age. The neck shield is longer than wide and bears longitudinal oval notches (punctation). On the elytra there are deepened longitudinal stripes.

The larvae are barrel-shaped, bear a light brown head capsule, are pale in color, and have no extremities.

Biology

The grain weevil belongs to the weevil family (Curculionidae), which are easily recognizable by their trunk-shaped elongated head. The grain weevil is flightless. The rather light-shy beetles mate already a few days after hatching. After a period of about four to ten days, the female begins to lay eggs. In the course of this, about 200 - 300 whitish shiny, 0.6 to 0.8 mm large eggs are laid in rates of up to three eggs per day. To do this, the female gnaws a short channel into a grain kernel, lays an egg inside, and closes the opening with a plug of secretion.

The development of the grain weevil from egg to newly hatched young beetle takes place entirely in the grain. They pass through four larval stages, finally pupating in their cereal grain. Development is strongly dependent on temperature and grain moisture. At optimum development temperatures of 25 °C to 29 °C, the development period of the beetles is about 34 days. Under domestic storage conditions, two to three generations per year are possible. At temperatures of 20 °C to 30 °C, the life span of the beetles is between five and twelve months. At lower temperatures of about 10 °C to 12 °C, all life processes slow down and the beetles can live up to more than two years. At low temperatures of about 5 °C, cold torpor occurs, temperatures of -10 °C are endured for up to two weeks, temperatures below -10 °C lead to cold death after only a short time. This relative cold tolerance allows the beetles to survive in unheated storage and in sheltered places in the open. At temperatures of about 38 °C, heat rigidity occurs; temperatures of 39 °C to 40 °C lead to heat death of the beetles. Reproduction is not possible at grain moisture levels below 10%, but such low moisture levels are almost unheard of in Europe. In empty storage rooms (without food), beetles are able to stay alive for the whole winter, and in summer for one to two months.

Damage symptoms

The larvae almost completely hollow out the grain. High infestation densities therefore result in enormous weight losses, and the cereal becomes musty. The feeding activity of the beetles and larvae and the associated metabolic processes lead to an increase in temperature and moisture in the stored material. As a result, fungi and bacteria colonize, the stored goods clump together, become inedible and finally take on a manure-like odor. Due to the formation of condensation water, the stored grain begins to grow out on the surface.

Host plants

The adult beetles feed on cereals as well as cereal products, such as flour, meal and semolina. The development of their larvae, on the other hand, is only possible in whole cereal grains. Besides the various cereals, development can also take place in corn, millet, buckwheat and pasta, but not in legume seeds, pumpkin seeds, sunflower seeds, or similar.

Distribution

The beetle originates from the Near East. In Europe, it was spread with the cultivation of cereals and is a crop follower. In Central Europe, it can overwinter outdoors only in sheltered places or in warehouses.

Propagation and transmission

Grain weevils are flightless because their hind wings are regressed. The pest is introduced when grain already infested with eggs and larvae is purchased. Infestation is often not discovered until beetles or the typical holes from which the adult beetles hatch are visible in the grain. To prevent spread and transmission, the grain should be inspected regularly.

Economic importance

A grain weevil infestation in the agricultural sector can have serious consequences on the harvest, as the infestation of fungi and bacteria caused by the activity of the grain weevil makes the grain inedible.

Prevention and control

Spread

  • Thorough cleaning of harvesting and transport machinery and storage areas (removal of junk and plaster debris, destruction of already infested supplies) before storing the crop.
  • Physical defense methods such as sealing windows and doors with rubber lips and insect screens, insulating ceilings and walls (avoiding condensation), plastering crevices, cracks, and porous walls, and avoiding floorboards and wood boarding with joints and gaps

Proof

  • Periodic temperature measurement: an increase in the temperature of the stored goods provides information about the presence of storage pests
  • Sampling of grain stocks by means of a grain pricker.
  • Sieving of the samples by means of a continuous sieve.
  • Using the alluvial test, infested, hollowed grain kernels floating on the water surface can be examined for grain weevil larvae.
  • Detection of larval feeding sounds by inserting a sensitive microphone into stored stores.

Control

  • Control of the grain weevil and its larval stages requires increased effort due to protected larval development inside the grain kernel.
  • Inert dusts (diatomaceous earth) and contact insecticides (see list of plant protection products approved in Austria) allow successful control of adult beetles if distributed appropriately in the stored material. However, eggs and larvae deposited in the grain are not captured and can continue to develop unhindered inside the grain.
  • Safe killing is achieved with gaseous agents (hydrogen phosphide) including inert gases (nitrogen, carbon dioxide).
  • Empty space treatment with an approved insecticide (see list of plant protection products approved in Austria) or heat treatment.
  • Cooling of the stored goods to 10-13 °C prevents further development.
  • Heating of the stored material to 60 °C for three minutes or freezing below -10 °C are possible but only effective for smaller batches due to the energy required.
  • Biological control is achieved, among other things, by releasing the storage wasp Lariophagus distinguendus. This insect finds the grain weevil larvae and pupae living hidden in the grain. This beneficial insect for controlling the grain weevil is currently not registered in Austria.

Last updated: 24.11.2021

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